The present invention relates to a continuous process for producing vinylaromatic polymers having a high degree of syndiotacticity. More particularly, the present invention relates to such a process that is capable of forming such polymers having a uniform small particle size in an efficient manner using a powder bed reactor.
It is previously known in the art to prepare vinylaromatic polymers having a high degree of syndiotacticity by the use of a liquid phase, powder bed reactor. This technique was disclosed in U.S. Pat. No. 5,037,907 wherein a vertical tank reactor equipped with a stirring agitator and filled with a particulated polydispersed material, especially particulated syndiotactic polystyrene, was employed. Vinyl aromatic monomer contained in a liquid reaction medium was dispersed through the system in the form of a coating on the particulates and caused to polymerize by use of a Ziegler-Natta catalyst, especially certain Group 4 metal complexes. The extremely large surface area of the particulated solid provides a large capacity for monomer to reside in the liquid phase, and the reaction is conducted at relatively modest temperatures from about 25.degree. C. to 85.degree. C. The process is referred to as a liquid phase polymerization since the monomer remains in the liquid phase during the polymerization. In theory, operation according to this prior art technique avoids the formation of large agglomerates of polymer because the liquid reaction medium remains dispersed on the surface of the particulated material. However, in practice the foregoing reactor design is still plagued by large particle or clump formation leading to poor product qualities, especially at low monomer conversions (less than 75 percent). In addition, the reactor design also achieves insufficient dispersive mixing, further contributing to poor product uniformity due to localized concentration of catalyst, monomer and/or diluent. These defects in the reactor design result in product molecular weight fluctuation and poor control of final product properties.
In U.S. Pat. No. 5,254,647 a process for preparing syndiotactic vinylaromatic polymers in a wiped surface reactor is disclosed. Such reactors allow improved operation by completely avoiding aggregate formation and by more thorough mixing. Desirably, the wiped surface reactor was employed to control polymerization until a powdered product was obtained, on the order of 10 to 20 percent polymerization. Continued polymerization was then conducted in a powder bed reactor such as the previously discussed vertical tank design. Although this scheme overcomes the problem of rapid polymerization in the early stages of the reaction under conditions of poor dispersive mixing, as occurs in the powder bed reactor, the use of two reactors introduces an increased capital expense. In addition, the first stage wiped surface reactor significantly limits the capacity of the system and thus the rate of polymer production. From a capital and process throughput point of view, it would be desirable to be able to conduct the polymerization entirely in a liquid phase, powder bed polymerizer.
For the teachings contained therein, the foregoing United States patents are herein incorporated by reference.